<body oninput="i.srcdoc=h.value+'<style>'+c.value+'</style><script>'+j.value+'</script>'"><style>textarea,iframe{width:100%;height:50%;}body{margin:0;}textarea{width: 33.33%;font-size:18px;padding:0.5em}</style><textarea placeholder="HTML" id="h"></textarea><textarea placeholder="CSS" id="c"></textarea><textarea placeholder="JS" id="j"></textarea><iframe id="i"></iframe><script>document.querySelectorAll("textarea").forEach((t)=>t.addEventListener("keydown",function(t){var e,s;"Tab"==t.key&&(t.preventDefault(),e=this.selectionStart,s=this.selectionEnd,this.value=this.value.substring(0,e)+"  "+this.value.substring(s),this.selectionStart=this.selectionEnd=e+1)}))</script></body>